Is there any form of security on paid addons
Nope. A lack of DRM goes with the free software culture surrounding Blender.

I obviously can't speak for why any particular addon authors choose to eschew DRM, but it's not uncommon at all. In fact, a number of paid addons are under the GPL and live in public repositories. Gaffer and RetopoFlow, for example.

Gaffer even goes so far as to encourage trying it out before buying; from the README:


  You can download it here to try it out. If you like it, please consider purchasing it from the Blender Market. $20 isn't much, and every sale puts a smile on my face and motivates me to keep developing it :)